This research will investigate how persons with congenital nystagmus maintain a stable and relatively clear visual world, despite the involuntary rapid oscillations of their eyes. In normal vision, extra-retinal signals accompany voluntary eye movements and help offset the retinal image changes that occur. In persons with nystagmus, the extra-retinal signals usually underrepresent eye movement amplitude (according to patients' reports of afterimage motion in the dark) and therefore supplementary mechanisms must contribute to perceived stability of the world. We will characterize precisely the amplitude and phase of extra- retinal signals for congenital nystagmus and determine whether the perceptual and motor systems receive the same or different signals. Parallel studies with normal subjects will evaluate whether extra- retinal information is available about an involuntary eye movement, optokinetic afternystagmus. In persons with nystagmus, thresholds are elevated dramatically for unreferenced motion in the meridian of eye movements. This insensitivity to (or discounting of) particular motions could contribute to perceptual stability. Unreferenced motion thresholds might be raised (at least in part) because movement of the target is obscured by image motions resulting from nystagmus. This possibility will be tested. In addition, we will determine how thresholds for relative motion and flicker, and the perceived velocities of suprathreshold motions, are affected. Visual acuity in persons with congenital nystagmus is better than expected from the typically large and rapid motions of the retinal image. Hypothetically, this occurs because the eyes slow momentarily as the fovea passes over a target of regard. This hypothesis will be tested by measuring acuity for targets presented at different phases of the nystagmus. In addition, the contributions to reduced acuity of 3 potential mechanisms will be determined: retinal image smearing, neural modification (an amblyopia), and variability in the accuracy of slowing the fovea on target. This research will contribute to understanding how stable and clear vision is maintained in persons with nystagmus, and also in normal vision. Further, it will indicate how visual functioning might be changed if congenital nystagmus is reduced, and so may lead to improved strategies for treatment.